Co clean-up catalyst, after treatment system and after treatment method

ABSTRACT

An after treatment method is disclosed. The after treatment method may include: operating an engine at a lean air/fuel ratio; calculating an amount of NH 3  stored in an SCR catalyst; calculating an amount of NOx which will flow into the SCR catalyst; determining whether conversion to a rich air/fuel ratio is desired; calculating, when the conversion to the rich air/fuel ratio is desired, a rich duration for which the rich air/fuel ratio is maintained and a target air/fuel ratio; and operating the engine at the target air/fuel ratio for the rich duration.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of non-provisional U.S. patentapplication Ser. No. 16/263,484, filed on Jan. 31, 2019, the entirecontents of which are incorporated herein by reference.

FIELD

The present disclosure relates to a CO clean-up catalyst (CUC), an aftertreatment system and an after treatment method.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Vehicles may be provided with at least one catalytic converter forreducing emission (EM) contained in an exhaust gas. The exhaust gasflowing out from an engine through an exhaust manifold is driven into acatalytic converter mounted at an exhaust pipe and is purified therein.After that, noise of the exhaust gas is decreased while passing througha muffler and then the exhaust gas is emitted into the air through atail pipe. The catalytic converter purifies the EM contained in theexhaust gas. In addition, a particulate filter for trapping particulatematter (PM) in the exhaust gas is mounted in the exhaust pipe.

A three-way catalyst (TWC) is one type of the catalytic converter andreacts with hydrocarbon (HC) compounds, carbon monoxide (CO) andnitrogen oxides (NOx), which are harmful components of the exhaust gas,to remove these compounds. The TWCs are mainly installed in gasolinevehicles, and Pt/Rh, Pd/Rh or Pt/Pd/Rh systems are used as the TWCs.

A lean-burn engine among gasoline engines may improve fuel efficiency byburning a lean air/fuel mixture. The lean-burn engine burns the leanair/fuel mixture, so air/fuel ratio of the exhaust gas is also lean.However, when the air/fuel ratio is lean, the TWC slips the NOx withoutreducing all of the NOx contained in the exhaust gas. Accordingly, avehicle equipped with the lean-burn engine may include a selectivecatalytic reduction (SCR) catalyst for purifying the NOx slipped fromthe TWC. The SCR catalyst used in the vehicle equipped with thelean-burn engine may be a passive type SCR catalyst.

When the air/fuel ratio is rich, the TWC reduces the NOx to produce NH₃and the NH₃ generated in the TWC is stored in the passive type SCRcatalyst. When the air/fuel ratio is lean, the passive type SCR catalystpurifies the NOx contained in the exhaust gas using the stored NH₃.

However, we have discovered that the TWC may slip CO at the richair/fuel ratio. The CO slipped from the TWC may not be purified and bedischarged to an outside of the vehicle. Therefore, an additionalcatalytic converter may be employed to purify the CO slipped from theTWC when the air/fuel ratio is adjusted to be rich for generating theNH₃.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the disclosure andtherefore it may contain information that does not form the prior artthat is already known to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

The present disclosure describes a CO clean-up catalyst (CUC) capable ofpurifying CO slipped from a three-way catalyst (TWC) at a rich air/fuelratio.

Another aspect of the present disclosure provides an after treatmentsystem having further advantages of efficiently purifying emissioncontained in an exhaust gas by sequentially disposing the TWC, aselective catalytic reduction (SCR) catalyst, and the CUC on an exhaustpipe.

A further aspect of the present disclosure provides an after treatmentmethod having further advantages of reducing the CO discharged to anoutside of a vehicle by efficiently controlling the after treatmentsystem.

A CO clean-up catalyst (CUC) according to an form of the presentdisclosure may include 0.2-1.5 wt % of Pt, 0-0.4 wt % of Pd, 0-0.4 wt %of Rh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂, 9.8-59.8 wt % of Al₂O₃, and0-20 wt % of an additive based on a total weight of the CUC.

The additive may include at least one of La, Zr, Mg, and Pr.

The weight ratio of the additive to the total weight of the CUC may be0-10 wt %.

An after treatment system according to another form of the presentdisclosure may include: an exhaust pipe through which an exhaust gasflows; a three-way catalyst (TWC) mounted on the exhaust pipe, oxidizingHC and CO contained in the exhaust gas at a lean air/fuel ratio, andgenerating NH₃ using NOx contained in the exhaust gas at a rich air/fuelratio; a selective catalytic reduction (SCR) catalyst mounted on theexhaust pipe at a downstream of the TWC, storing the NH₃ generated inthe TWC, and reducing the NOx contained in the exhaust gas using thestored NH₃; and a CO clean-up catalyst (CUC) mounted on the exhaust pipeat a downstream of the SCR catalyst and purifying the CO slipped fromthe TWC or contained in the exhaust gas at the rich air/fuel ratio.

The after treatment system may further include a particulate filterdisposed between the TWC and the SCR catalyst, wherein the particulatefilter traps particulate matter in the exhaust gas.

The after treatment system may further include an additional TWC or anammonia production catalyst (APC) disposed between the TWC and the SCRcatalyst, wherein the additional TWC or the APC further generates NH₃using the NOx contained in the exhaust gas at the rich air/fuel ratio.

In one aspect, the APC may include 0.4-0.9 wt % of Pt, 0.057-0.3 wt % ofPd, 0.03-0.1 wt % of Rh, 5.0-15.0 wt % of Ba, 10-30 wt % of CeO₂,48.7-84.513 wt % of a composite of MgO and Al₂O₃, and 0-5 wt % of anadditive based on a total weight of the APC.

In another aspect, the APC may include 0.4-0.9 wt % of Pt, 0.057-0.3 wt% of Pd, 0.03-0.1 wt % of Rh, 5.0-15.0 wt % of Ba, 10-25 wt % of CeO₂,48.7-79.513 wt % of a composite of MgO and Al₂O₃, and 0-10 wt % of anadditive based on a total weight of the APC.

A lean NOx trap (LNT) catalyst may be further coated in the TWC or theadditional TWC, wherein the LNT catalyst absorbs the NOx contained inthe exhaust gas at the lean air/fuel ratio, and releases and reduces theabsorbed NOx into N₂ gas or the NH₃ at the rich air/fuel ratio.

The CUC may include 0.2-1.5 wt % of Pt, 0-0.4 wt % of Pd, 0-0.4 wt % ofRh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂, 9.8-59.8 wt % of Al₂O₃, and0-20 wt % of an additive based on a total weight of the CUC.

The additive may include at least one of La, Zr, Mg, and Pr.

An after treatment method according to other form of the presentdisclosure controls an after treatment system equipped with a three-waycatalyst (TWC), a selective catalytic reduction (SCR) catalyst, and a COclean-up catalyst (CUC) on an exhaust pipe through which an exhaust gasflows.

The after treatment method may include: by a controller, operating anengine at a lean air/fuel ratio; calculating an amount of NH₃ stored inthe SCR catalyst; calculating an amount of NOx which will flow into theSCR catalyst; determining whether conversion to a rich air/fuel ratio isdesired; calculating, when the conversion to the rich air/fuel ratio isdesired, a rich duration for which the rich air/fuel ratio is maintainedand a target air/fuel ratio; and operating the engine at the targetair/fuel ratio for the rich duration.

The rich duration may be calculated according to the target air/fuelratio and a temperature of the CUC.

The rich duration may be calculated such that a slip amount of COaccumulated at a downstream of the CUC for the rich duration is smallerthan or equal to a predetermined value if the engine is operated at thetarget air/fuel ratio for the rich duration.

The after treatment method may further include a particulate filterdisposed between the TWC and the SCR catalyst, wherein the particulatefilter traps particulate matter in the exhaust gas.

The after treatment method may further include an additional TWC or anammonia production catalyst (APC) disposed between the TWC and the SCRcatalyst, wherein the additional TWC or the APC further generates NH₃using the NOx contained in the exhaust gas at the rich air/fuel ratio.

The CUC may include 0.2-1.5 wt % of Pt, 0-0.4 wt % of Pd, 0-0.4 wt % ofRh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂, 9.8-59.8 wt % of Al₂O₃, and0-20 wt % of an additive based on a total weight of the CUC.

The additive may include at least one of La, Zr, Mg, and Pr.

According to forms of the present disclosure, the CO slipped from theTWC when the air/fuel ratio is adjusted to be rich for generating theNH₃ may be purified by disposing the CUC at a rear end portion of theafter treatment system.

In addition, since the rich duration is adjusted according to thetemperature of the CUC and the target air/fuel ratio, it is possible toincrease the NH₃ generation while reducing the CO discharged to theoutside of the vehicle.

In addition, other effects of the forms of the present disclosure shouldbe directly or implicitly described in the description provided herein.Various effects predicted according to the forms of the presentdisclosure will be disclosed in the description provided herein.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 2 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 3 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 4 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 5 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 6 is a schematic diagram of an after treatment system according toa form of the present disclosure;

FIG. 7 illustrates an amount of NH₃ generated in the TWC at a richair/fuel ratio, and amounts of CO slipped at a rear end portion of anafter treatment system at a rich air/fuel ratio when a CUC is appliedand when a CUC is not applied;

FIG. 8 is a graph illustrating CO purification capacity according tocompositions of a CUC;

FIG. 9 is a block diagram of an after treatment system according to aform of the present disclosure; and

FIG. 10 is a flowchart of an after treatment method according to a formof the present disclosure.

It should be understood that the above-referenced drawings are notnecessarily to scale, presenting a somewhat simplified representation ofvarious features illustrative of the basic principles of the disclosure.The specific design features of the present disclosure, including, forexample, specific dimensions, orientations, locations, and shapes, willbe determined in part by the particular intended application and useenvironment.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

The terminology used herein is not intended to be limiting of thepresent disclosure. As used herein, the singular forms “a,” “an,” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items. The term “coupled” denotes a physicalrelationship between two components whereby the components are eitherdirectly connected to one another or indirectly connected via one ormore intermediary components.

It is understood that the term “vehicle,” “vehicular,” “car,” or othersimilar term as used herein is inclusive of motor vehicles, in general,such as passenger automobiles including sports utility vehicles (SUV),buses, trucks, various commercial vehicles, watercraft including avariety of boats and ships, aircraft, and the like, and includes hybridvehicles, electric vehicles, hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum).

Additionally, it is understood that one or more of the below methods, oraspects thereof, may be executed by at least one controller. The term“controller” may refer to a hardware device that includes a memory and aprocessor. The memory is configured to store program instructions, andthe processor is specifically programmed to execute the programinstructions to perform one or more processes which are describedfurther below. Moreover, it is understood that the below methods may beexecuted by a system comprising the controller, as described in detailbelow.

Furthermore, the controller of the present disclosure may be embodied asnon-transitory computer readable media containing executable programinstructions executed by a processor or the like. Examples of thecomputer readable mediums include, but are not limited to, ROM, RAM,compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives,smart cards, and optical data storage devices. The computer readablerecording medium can also be distributed throughout a computer networkso that the program instructions are stored and executed in adistributed fashion, e.g., by a telematics server or a Controller AreaNetwork (CAN).

Hereinafter, forms of the present disclosure will be described in detailwith reference to the accompanying drawings.

FIG. 1 is a schematic diagram of an after treatment system according toan form of the present disclosure.

As shown in FIG. 1, an after treatment system according to an form ofthe present disclosure includes an engine 10, an exhaust pipe 20, athree-way catalyst (TWC) 30, a particulate filter (GPF) 40, a selectivecatalytic reduction (SCR) catalyst 50, and a CO clean-up catalyst (CUC)60.

The engine 10 burns an air/fuel mixture to convert chemical energy intomechanical energy. The engine 10 is connected to an intake manifold 16to flow air into a combustion chamber 12. An exhaust gas generated incombustion process is collected in an exhaust manifold 18 and then flowsout from the engine 10. The combustion chamber 12 is equipped with aspark plug 14 to ignite the air/fuel mixture within the combustionchamber 12. The engine 10 may be a gasoline engine. Depending on typesof gasoline engines, fuel may be directly injected into the combustionchamber 12 or the air/fuel mixture may be supplied to the combustionchamber 12 via the intake manifold 16.

The exhaust pipe 20 is connected to the exhaust manifold 18 to dischargethe exhaust gas to an outside of the vehicle. The exhaust pipe 20 isequipped with the TWC 30, the particulate filter 40, the SCR catalyst50, and the CUC 60 to purify or remove emission and particulate mattercontained in the exhaust gas.

The TWC 30 is disposed on the exhaust pipe 20 through which the exhaustgas discharged from the engine 10 flows, and harmful materials includingCO, HC, and NOx contained in the exhaust gas are converted into harmlesscomponents by an oxidation-reaction reaction in the TWC 30.Particularly, the TWC 30 can reduce the NOx contained in the exhaust gasinto NH₃ at a rich air/fuel ratio (AFR). At this time, the TWC 30 maynot purify the CO in the exhaust gas sufficiently and may slip the COtherefrom. In addition, the TWC 30 oxidizes the CO and the HC containedin the exhaust gas at a lean AFR.

The particulate filter 40 is mounted on the exhaust pipe 20 at adownstream of the TWC 30, traps the particulate matter contained in theexhaust gas, and burns the trapped particulate matter. The particulatefilter 40 is provided with inlet cells and outlet cells alternatelydisposed in a housing, and a wall is disposed between the inlet cell andthe outlet cell. The inlet cell has an end that is opened and the otherend that is blocked, and the outlet cell has an end that is blocked andthe other end that is opened. The exhaust gas flows into the particulatefilter 40 through the opened end of the inlet cell, flows to the outletcell through the wall, and flows out to an outside of the particulatefilter 40 through the opened end of the outlet cell. When the exhaustgas passes through the wall, the particulate filter contained in theexhaust gas does not pass through the wall and remains in the inletcell.

The SCR catalyst 50 is mounted on the exhaust pipe 20 at a downstream ofthe particulate filter 40. The SCR catalyst 50 stores the NH₃ generatedin the TWC 30 at the rich AFR and reduces the NOx contained in theexhaust gas using the stored NH₃ at the lean AFR. This type of the SCRcatalyst 50 may be referred to as a passive type SCR catalyst 50.

The SCR catalyst 50 may include one or a combination of a zeolitecatalyst and a metal catalyst supported in a porous Al₂O₃. At least oneof Cu, Pt, Mn, Fe, Co, Ni, Zn, Cs and Ga may be ion-exchanged in thezeolite catalyst. In the metal catalyst supported in the porous Al₂O₃,at least one metal among Pt, Pd, Rh, Ir, Ru, W, Cr, Mn, Fe, Co, Cu, Znand Ag may be supported in the porous Al₂O₃.

The CUC 60 is mounted on the exhaust pipe 20 at a downstream of the SCRcatalyst 50. The CUC 60 purifies the CO contained in the exhaust gas.Particularly, when the AFR is rich, the CO can be slipped from the TWC30, and the CUC 60 is mounted at the rear end portion of the aftertreatment system to prevent or inhibit CO from discharging to theoutside of the vehicle. The CUC 60 includes Pt, Pd, Rh, and Ba supportedin CeO₂ and Al₂O₃.

In one aspect, the CUC 60 includes 0.2-1.5 wt % of Pt, 0-0.4 wt % of Pd,0-0.4 wt % of Rh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂, 9.8-59.8 wt % ofAl₂O₃, and 0-10 wt % of an additive based on a total weight of the CUC60.

In another aspect, the CUC 60 includes 0.2-1.5 wt % of Pt, 0-0.4 wt % ofPd, 0-0.4 wt % of Rh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂, 9.8-59.8 wt% of Al₂O₃, and 0-20 wt % of an additive based on a total weight of theCUC 60.

The additive is added for improving performance of the CeO₂ and theAl₂O₃ and includes at least one of La, Zr, Mg and Pr.

The CUC 60 may, in one form, be mainly composed of Pt/CeO₂. Here, the Ptfunctions to oxidize the CO, and the CeO₂ contains an oxygen storagematerial to help oxidation of the CO at low temperature at the lean AFR.Pd/Al₂O₃ also plays a role similar to the Pt/CeO₂, but an amount of thePt/CeO₂ may be greater than that of the Pd/Al₂O₃ in order to improveoxidation ability at the low temperature.

The Ba contained in the CUC 60 functions to remove a small amount of theNOx that is not removed from the SCR catalyst 50 when the AFR is rich.

The Rh included in the CUC 60 is intended to promote reduction of theNOx when the AFR is rich.

The exhaust pipe 20 may be equipped with a plurality of sensors 32, 34,62, and 64 for detecting the AFR of the exhaust gas and operation of thecatalysts 30, 40, 50 and 60.

A first oxygen sensor 32 is mounted on the exhaust pipe 20 at anupstream of the TWC 30, detects 02 concentration in the exhaust gas atthe upstream of the TWC 30, and transmits a signal corresponding theretoto a controller 90.

A second oxygen sensor 34 is mounted on the exhaust pipe 20 at thedownstream of the TWC 30, detects 02 concentration in the exhaust gas atthe downstream of the TWC 30, and transmits a signal correspondingthereto to the controller 90.

A first temperature sensor 62 is mounted on the exhaust pipe 20 at anupstream of the SCR catalyst 50, detects a temperature of the exhaustgas at the upstream of the SCR catalyst 50, and transmits a signalcorresponding thereto to the controller 90.

A second temperature sensor 64 is mounted on the exhaust pipe 20 at thedownstream of the SCR catalyst 50, detects the temperature of theexhaust gas at the downstream of the SCR catalyst 50, and transmits asignal corresponding thereto to the controller 90.

In addition to the sensors 32, 34, 62, and 64 described herein, theafter treatment system may further include various sensors. For example,additional temperature sensors can be mounted on the exhaust pipe 20 atthe upstream and the downstream of the TWC 30 to detect the temperatureof the exhaust gas at the upstream and the downstream of the TWC 30,respectively. In addition, as shown in FIG. 9, the after treatmentsystem may further include an air flowmeter 66. Further, the aftertreatment system may further include a NOx sensor, an HC sensor or a COsensor mounted on the exhaust pipe 20, and concentration of emissioncontained in the exhaust gas can be detected via these sensors.

The controller 90 is electrically connected to the sensors 32, 34, 62,64 and 66 to receive the signals corresponding to the detected values bythe sensors 32, 34, 62, 64 and 66, and determines driving condition ofthe vehicle, the AFR, and the temperatures of the catalysts 30, 40, 50,and 60 based on the signals. The controller 90 can control ignitiontiming, fuel injection timing, fuel amount, etc., by controlling theengine 10 based on the determination results. The controller 90 may beimplemented with at least one processor executed by a predeterminedprogram and the predetermined program may be programmed to perform eachstep of an after treatment method according to an form of the presentdisclosure.

FIG. 2 is a schematic diagram of an after treatment system according toanother form of the present disclosure. The after treatment system shownin FIG. 2 is a modification of the after treatment system shown in FIG.1.

As shown in FIG. 2, the after treatment system according to another formof the present disclosure is equipped with the TWC 30, an additional TWC70, the particulate filter 40, the SCR catalyst 50, and the CUC 60sequentially disposed on the exhaust pipe 20. Since the TWC 30, theparticulate filter 40, the SCR catalyst 50, and the CUC 60 are describedabove, a detailed description thereof will be omitted.

The additional TWC 70 is disposed on the exhaust pipe 20 between the TWC30 and the particulate filter 40. The additional TWC 70 basicallyperforms the same function as the TWC 30. In other words, the additionalTWC 70 converts harmful materials including the CO, the HC, and the NOxcontained in the exhaust gas into harmless components byoxidation-reaction reaction. In addition, the additional TWC 70increases the amount of NH₃ supplied to the SCR catalyst 50 by reducingthe NOx contained in the exhaust gas into NH₃ when the AFR is rich. Inaddition, a lean NOx trap (LNT) catalyst may be further coated in theadditional TWC 70 and the TWC 30. The LNT catalyst absorbs the NOx atthe lean AFR, and releases the absorbed NOx and reduces the released NOxinto the N₂ gas or the NH₃ at the rich AFR. Thus, the LNT catalystassists the TWC having low NOx purification performance at the lean AFR.The LNT catalyst includes at least one or a combination of an alkalimetal including K, Na, Li, and Cs, an alkaline earth metal including Baand Ca, a rare earth metal including Ir and La, and a noble metalincluding Pt, Pd, and Rh.

FIG. 3 is a schematic diagram of an after treatment system according toother form of the present disclosure. The after treatment system shownin FIG. 3 uses a TWC 30′ coated with the LNT catalyst instead of the TWC30 in the after treatment system shown in FIG. 1. Since the aftertreatment system shown in FIG. 1 and the TWC 30′ coated with the LNTcatalyst are described above, a detailed description thereof will beomitted.

FIG. 4 is a schematic diagram of an after treatment system according toother form of the present disclosure. The after treatment system shownin FIG. 4 is a modification of the after treatment system shown in FIG.1.

As shown in FIG. 4, the after treatment system according to other formof the present disclosure is equipped with the TWC 30, an ammoniaproduction catalyst (APU) 80, the SCR catalyst 50, and the CUC 60sequentially disposed on the exhaust pipe 20. Since the TWC 30, the SCRcatalyst 50, and the CUC 60 are described above, a detailed descriptionthereof will be omitted.

The APC 80 is disposed at the exhaust pipe 20 at the downstream of theTWC 30. The APC 80 stores the NOx contained in the exhaust gas at thelean AFR, and generates H₂ to release the stored NOx and generates NH₃using the released NOx and the generated H₂.

In one aspect, the APC 80 includes 0.4-0.9 wt % of Pt, 0.057-0.3 wt % ofPd, 0.03-0.1 wt % of Rh, 5.0-15.0 wt % of Ba, 10-30 wt % of CeO₂,48.7-84.513 wt % of a composite of MgO and Al₂O₃, and 0-5 wt % of anadditive based on a total weight of the APC 40.

In another aspect, the APC 80 includes 0.4-0.9 wt % of Pt, 0.057-0.3 wt% of Pd, 0.03-0.1 wt % of Rh, 5.0-15.0 wt % of Ba, 10-25 wt % of CeO₂,48.7-79.513 wt % of the composite of MgO and Al₂O₃, and 0-10 wt % of theadditive based on the total weight of the APC.

The additive is added for the performance improvement of CeO₂ and Al₂O₃and includes at least one of La, Zr, Mg and Pr.

The Pt contained in the APC 80 functions to oxidize the NOx for the APC80 to store the NOx. In addition, the Pt increases an amount of H₂generated in the APC 80.

The Pd contained in the APC 80 improves heat resistance of the APC 80.Since the APC 80 is disposed close to the engine 10, a temperature ofthe APC 80 can rise to 950° C. Therefore, the Pd is added in the APC 80to improve heat resistance.

In order to increase the NH₃ generation and the H₂ generation, a weightratio of the Pt to the Pd in the APC 80 may be 3:1-7:1. In one aspect,the weight ratio of the Pt to the Pd in the APC 80 may be 3:1-5:1.

The Rh contained in the APC 80 purifies the NOx contained in the exhaustgas at a stoichiometric AFR.

The Ba and the CeO₂ contained in the APC 80 is configured to store theNOx in nitrate form.

In addition, the CeO₂ increases H₂ generation. However, if the APC 80contains large amounts of the CeO₂, the generated NH₃ can be reoxidized.Thus, the APC 80 may include 10-30 wt % of CeO₂ based on a total weightof the APC 80.

The composite of MgO and Al₂O₃ contained in the APC 80 may function as asubstrate. The composite of MgO and Al₂O₃ may include 15-25 wt % of MgObased on a total weight of the composite of MgO and Al₂O₃. The MgOenhances thermal stability of the Ba.

FIG. 5 is a schematic diagram of an after treatment system according toother form of the present disclosure. The after treatment system shownin FIG. 5 is a modification of the after treatment system shown in FIG.4.

As shown in FIG. 5, the after treatment system according to other formof the present disclosure is equipped with the TWC 30, the particulatefilter 40, the APC 80, the SCR catalyst 50, and the CUC 60 sequentiallydisposed on the exhaust pipe 20. Since the TWC 30, the particulatefilter 40, the APC 80, the SCR catalyst 50, and the CUC 60 are describedabove, a detailed description thereof will be omitted.

FIG. 6 is a schematic diagram of an after treatment system according toother form of the present disclosure. The after treatment system shownin FIG. 6 is a modification of the after treatment system shown in FIG.4.

As shown in FIG. 6, the after treatment system according to other formof the present disclosure is equipped with the TWC 30, the APC 80, theparticulate filter 40, the SCR catalyst 50, and the CUC 60 sequentiallydisposed on the exhaust pipe 20. Since the TWC 30, the APC 80, theparticulate filter 40, the SCR catalyst 50, and the CUC 60 are describedabove, a detailed description thereof will be omitted.

According to the forms of the present disclosure, catalysts (e.g., theTWC, the additional TWC, the LNT, the APC, etc.) which can generate theNH₃ at the rich AFR is disposed at a front end portion of the aftertreatment system, the SCR catalyst which stores the NH₃ and purifies theNOx using the stored NH₃ is disposed at a middle portion of the aftertreatment system, and the CUC is disposed at the rear end portion of theafter treatment system. Therefore, purifying performance for theemissions contained in the exhaust gas can be improved.

EXAMPLES

HHereinafter, the performance of the CUC 60 will be described throughvarious examples and comparative examples. The compositions of variousexamples and comparative examples are shown in [Table 1].

TABLE 1 addi- Pt Pd Rh BaO CeO₂ Al₂O₃ tive (wt %) (wt %) (wt %) (wt %)(wt %) (wt %) (wt %) Form1 0.9 0.1 0 0 80 17.9 1.1 Form2 0.9 0.1 0 0 4057.6 1.4 Form3 0.8 0.1   0.1   4.0 40 53.6 1.4 Com- 0   2.0 0 0  0 96.31.7 parative Exam- ple1 Com- without catalyst coating (bare substrate)parative Exam- ple2

(Test Method)

Powders of the CeO₂ and the Al₂O₃ are mixed and the noble metal (Pt, Pd,Rh, etc.) is fixed to the CeO₂ and the Al₂O₃ in slurry state by ionexchange method. The catalyst is prepared by calcining and burning theslurry.

The TWC 30, the GPF 40, and the SCR catalyst 50 are sequentiallydisposed on the exhaust pipe 20. In Example (or Form) 1 to Example (orForm) 3 and Comparative Example 1, the CUC 60 is disposed on the exhaustpipe 20 at the downstream of the SCR catalyst 50, and in ComparativeExample 2, a bare substrate in which the catalyst is not coated isdisposed on the exhaust pipe 20 at the downstream of the SCR catalyst50. After that, 2.0 L, 4-cylinders, lean-burn gasoline engine isconnected to the exhaust pipe 20 and aging treatment is performed. Theaging treatment is carried out at 1,000° C. on the TWC 30 basis, 720° C.on the SCR catalyst 50 basis, and 680° C. on the CUC 60 basis for 50hours.

The lean AFR (λ=1.8) is maintained for 5 minutes at an engine speed of2,000 rpm to cause the entire after treatment system to be leanatmosphere. Thereafter, an amount of NH₃ accumulated at the downstreamof the TWC 30 over time, and an amount of CO accumulated at thedownstream of the CUC 60 or the bare substrate in which the catalyst isnot coated over time are measured while the rich AFR (λ=0.97) ismaintained at the engine speed of 2,000 rpm.

FIG. 7 illustrates an amount of NH₃ generated in the TWC at a richair/fuel ratio, and amounts of CO slipped at a rear end portion of anafter treatment system at a rich air/fuel ratio when a CUC is appliedand when a CUC is not applied.

In FIG. 7, a thick solid line represents the amount of the NH₃accumulated at the downstream of the TWC 30, and a thin solid linerepresents the amount of the CO accumulated at the downstream of the CUC60 in Example 1 (i.e., when the CUC is applied), and a thin dotted linerepresents the amount of the CO accumulated at the downstream of thebare substrate in which the catalyst is not coated in ComparativeExample 2 (i.e., when the CUC is not applied).

As shown in FIG. 7, since the TWC 30 has an oxygen storage material,there is a delay time T1 of approximately 4 seconds from beginning ofthe rich AFR to beginning of NH₃ generation. When the delay time T1elapses, O₂ stored in the oxygen storage material of the TWC 30 ispurged, so that NH₃ is generated.

Also, when the O₂ stored in the oxygen storage material of the TWC 30 ispurged, the CO begins to slip (see the dotted line in FIG. 7). Since theCUC is not applied in Comparative Example 2, the amount of the COaccumulated at the downstream of the bare substrate over timecorresponds to the amount of the CO slipped from the TWC 30 over time.

In Example 1, the CO begins to slip from the CUC 60 when a delay time T2elapses after the beginning of the rich AFR. That is, the CUC 60 has theability to purify the CO slipped from the TWC 30 during the delay timeT2, and does not adequately purify the CO slipped from the TWC 30 afterthe delay time T2. The delay time T2 is about 7 to 8 seconds.

In Example 1, if the number of times to enter the rich AFR is increasedwhile reducing the rich duration in which the AFR is kept to be rich,the NOx contained in the exhaust gas can be sufficiently oxidized whilereducing emission of the CO. For example, if the rich duration in whichthe AFR is kept to be rich is set to 9 seconds and the number of timesto enter the rich AFR is set to 11 times in the after treatment systemof Example 1, the TWC 30 generates 0.78 g of the NH₃ for 99 seconds (9seconds*11 times) and 0.78 g of the NH₃ can purify about 2.1 g of theNOx. In this case, the amount of the CO slipped from the CUC 60 is about0.1 g. Therefore, it is desirable to increase the number of times toenter the rich AFR while reducing the rich duration to reduce the amountof the CO slipped from the CUC 60 while generating sufficient amount ofthe NH₃ in the TWC 30. The rich duration may be larger than the delaytime T1 of the TWC 30 and slightly larger than the delay time T2 of theCUC 60.

FIG. 8 is a graph illustrating CO purification capacity according tocompositions of a CUC. FIG. 8 is a graph illustrating the COpurification capacity according to the temperature and the compositionsof the CUC. A temperature range (200° C.-300° C.) of the CUC 60 shown inFIG. 8 is a temperature range that the temperature of the CUC 60 of theafter treatment system shown in FIG. 1 can be at a slightly rich AFR(λ=0.97).

As shown in FIG. 8, the CO purification capacity is 0 g/L regardless ofthe temperature of the CUC 60 in Comparative Example 2 in which the CUC60 is not applied. That is, if the CUC 60 is not applied at the rear endportion of the after treatment system, the CO slipped from the TWC 30 atthe rich AFR is not purified but discharged to the outside of thevehicle.

Comparing Example 1 to Example 3 with Comparative Example 1, aPt/CeO₂-based CUC 60 has a higher CO purification capacity at the sametemperature of the CUC than a Pd/Al₂O₃-based CUC 60. That is, increasingthe amount of the Pt/CeO₂ in the CUC 60 can improve the CO purifyingability.

Comparing Example 1 with Example 2 and Example 3, if the amount of thenoble metal (Pt, Pd, Rh) is the same, increasing the amount of the CeO₂improves the CO purifying ability.

Also, comparing Example 2 with Example 3, even if the Ba is added forstoring NOx, the CO purification ability is not substantially reduced ifthe amount of the CeO₂ and the amount of the noble metal are maintained.

On the other hand, the CO purifying capacity of the CUC 60 increaseswith increasing temperature of the CUC 60 when the temperature of theCUC 60 is 200° C.-300° C.

Resultantly, it can be seen that increasing the amount of the Pt and theCeO₂ contained in the CUC 60 is advantageous for CO removal. Inaddition, it can be seen that, even if the Ba and the like are added tothe CUC 60 for additional functions of the CUC 60, the purificationability of the CO is hardly reduced as long as the amount of the noblemetal and the CeO₂ is maintained.

FIG. 9 is a block diagram of an after treatment system according to anform of the present disclosure.

FIG. 9 illustrates a simple example of inputs and outputs of thecontroller 90 to implement the after treatment system according to formsof the present disclosure. It should be understood that the inputs andthe outputs of the controller 90 according to forms of the presentdisclosure are not limited to the example illustrated in FIG. 9.

As shown in FIG. 9, the controller 90 is electrically connected to thefirst and second oxygen sensors 32 and 34, the first and secondtemperature sensors 62 and a 64, and the air flowmeter 66, and receivesthe signals corresponding to the values detected by the sensors 32, 34,62, 64 and 66.

The first oxygen sensor 32 detects the concentration of the 02 containedin the exhaust gas at the upstream of the TWC 30 and transmits thesignal corresponding thereto to the controller 90. The second oxygensensor 34 detects the concentration of oxygen contained in the exhaustgas at the downstream of the TWC 30 and transmits the signalcorresponding thereto to the controller 90. The controller 90 candetermine whether the TWC 30 is operating normally based on the signalsof the first and second oxygen sensors 32 and 34, and performs air/fuelratio control of the engine 10.

The first temperature sensor 62 detects the temperature of the exhaustgas at the upstream of the SCR catalyst 50 and transmits the signalcorresponding thereto to the controller 90. The second temperaturesensor 64 detects the temperature of the exhaust gas at the downstreamof the SCR catalyst 50 and transmits the signal corresponding thereto tothe controller 90. The controller 90 can calculate the temperatures ofthe TWC 30, the particulate filter 40, the SCR catalyst 50, and the CUC60 based on the signals of the first and second temperature sensors 62and 64.

The air flowmeter 66 is mounted on an intake pipe or an intake duct todetect the amount of the air flowing into the intake system, andtransmits the signal corresponding thereto to the controller 90.

The controller 90 controls operation of the engine 10 based on thevalues detected by the sensors 32, 34, 62, 64 and 66. That is, thecontroller 90 can adjust the fuel injection amount to adjust the targetAFR, and can delay the ignition timing for warming up the catalysts 30,40, 50 and 60.

FIG. 10 is a flowchart of an after treatment method according to an formof the present disclosure.

As shown in FIG. 10, when the engine 10 is started at step S110, thecontroller 90 calculates the temperatures of the catalysts 30, 40, 50and 60. In order to carry out the after treatment method according tothe form of the present disclosure, the catalysts 30, 40, 50 and 60 areactivated. Accordingly, the controller 90 warms up the catalysts 30, 40,50 and 60 at step S120 if the catalysts 30, 40, 50 and 60 are notactivated. That is, the ignition timing is delayed or the amount of thefuel injection is increased to increase the temperature of the exhaustgas.

When warming-up of the catalyst 30, 40, 50, and 60 is completed, thecontroller 90 operates the engine 10 at the lean AFR at step S130.Therefore, the TWC 30 purifies the CO and the HC contained in theexhaust gas and the particulate filter 40 collects the particulatematter contained in the exhaust gas.

The controller 90 calculates the amount of the NH₃ stored in the SCRcatalyst 50 at step S140. That is, the amount of the NH₃ stored in theSCR catalyst 50 is calculated based on operation history of the engine10, temperature history of the SCR catalyst 50, and the like.

Also, the controller 90 calculates the amount of NOx to be flowed intothe SCR catalyst 50 at step S150. The amount of the NOx generated in theengine 10 is calculated based on a combustion state (e.g., a combustiontemperature, a combustion pressure, the air amount, the fuel amount,etc.) of the engine 10, and the amount of the NOx slipped from the TWC10 is calculated based on the AFR of the exhaust gas, the temperature ofthe TWC 30, etc.

Thereafter, the controller 90 determines whether the SCR catalyst 50 canpurify the NOx at step S160. That is, it is determined whether theamount of the NH₃ stored in the SCR catalyst 50 is sufficient to purifythe NOx flowing into the SCR catalyst 50.

If the SCR catalyst 50 can purify the NOx at the step S160, thecontroller 90 terminates the after treatment method according to theform of the present disclosure. In this case, the engine 10 is operatedat the lean AFR.

If the SCR catalyst 50 is not capable of purifying the NOx (i.e., theamount of the NH₃ stored in the SCR catalyst 50 is not sufficient topurify the NOx flowing into the SCR catalyst 50) at the step S160, thecontroller 90 calculates the rich duration and the target AFR forgenerating the NH₃ at step S170. As mentioned earlier, it is desirableto increase the number of times to enter the rich AFR while reducing therich duration to reduce the amount of the CO slipped from the CUC 60while generating the sufficient NH₃ in the TWC 30. Therefore, the richduration can be calculated so that the slip amount of the CO accumulatedat the downstream of the CUC 60 is below a predetermined value when theengine is operated for the rich duration at the target AFR. In addition,the target AFR can be set by a person of an ordinary skill in the art toimprove the performance of the after treatment system while reducingfuel consumption. For example, the target AFR may be 0.97 that isslightly rich AFR, but is not limited thereto. Also, as shown in FIG. 8,the CO purification capacity varies with the temperature of the CUC 60.Therefore, the rich duration can be calculated according to the targetAFR and the temperature of the CUC 60. The rich duration may be largerthan the delay time T1 of the TWC 30 and slightly larger than the delaytime T2 of the CUC 60.

When the rich duration and the target AFR are calculated at the stepS170, the controller 90 operates the engine 10 at the target AFR for therich duration at step S180. That is, by driving the engine 10 at therich AFR for the rich duration, the TWC 30 generates the NH₃ whilereducing the amount of the CO slipped from the CUC 60.

After performing the step S180, the controller 90 returns to the stepS130 and operates the engine 10 at the lean AFR. Even when the engine 10is operated at the target AFR for the rich duration, the NH₃ generationmay not be sufficient to purify the NOx that flows into the SCR catalyst50 (e.g., the NOx amount that flows into the SCR catalyst 50 is greaterthan the amount of the NOx that can be purified by the NH₃ generationfor the rich duration). Accordingly, the controller 90 operates theengine at the lean AFR for a predetermined time, and then performs thestep S140 to the step S180 again. If the sufficient NH₃ is generated topurify the NOx flowing into the SCR catalyst 50, then the aftertreatment method will be terminated at the step S160.

While this disclosure has been described in connection with what ispresently considered to be practical forms, it is to be understood thatthe disclosure is not limited, but covers various modifications andequivalent arrangements included within the spirit and scope of theaspects of the disclosure.

What is claimed is:
 1. An after treatment system comprising: an exhaustpipe through which an exhaust gas flows; a three-way catalyst (TWC)mounted on the exhaust pipe, oxidizing HC and CO contained in theexhaust gas at a lean air/fuel ratio, and generating NH₃ using NOxcontained in the exhaust gas at a rich air/fuel ratio; a selectivecatalytic reduction (SCR) catalyst mounted on the exhaust pipe at adownstream of the TWC, storing the NH₃ generated in the TWC, andreducing the NOx contained in the exhaust gas using the stored NH₃; a COclean-up catalyst (CUC) mounted on the exhaust pipe downstream of theSCR catalyst and purifying the CO slipped from the TWC or contained inthe exhaust gas at the rich air/fuel ratio; and an additional TWC or anammonia production catalyst (APC) disposed between the TWC and the SCRcatalyst, wherein the additional TWC or the APC further generates NH₃using the NOx contained in the exhaust gas at the rich air/fuel ratio.2. The after treatment system of claim 1, further comprising aparticulate filter disposed between the TWC and the SCR catalyst,wherein the particulate filter traps particulate matter in the exhaustgas.
 3. The after treatment system of claim 1, wherein the APC includes0.4-0.9 wt % of Pt, 0.057-0.3 wt % of Pd, 0.03-0.1 wt % of Rh, 5.0-15.0wt % of Ba, 10-30 wt % of CeO₂, 48.7-84.513 wt % of a composite of MgOand Al₂O₃, and 0-5 wt % of an additive based on a total weight of theAPC.
 4. The after treatment system of claim 1, wherein the APC includes0.4-0.9 wt % of Pt, 0.057-0.3 wt % of Pd, 0.03-0.1 wt % of Rh, 5.0-15.0wt % of Ba, 10-25 wt % of CeO₂, 48.7-79.513 wt % of a composite of MgOand Al₂O₃, and 0-10 wt % of an additive based on a total weight of theAPC.
 5. The after treatment system of claim 1, wherein a lean NOx trap(LNT) catalyst is further coated in the TWC or the additional TWC, andwherein the LNT catalyst absorbs the NOx contained in the exhaust gas atthe lean air/fuel ratio, and releases and reduces the absorbed NOx intoN₂ gas or the NH₃ at the rich air/fuel ratio.
 6. The after treatmentsystem of claim 1, wherein the CUC includes 0.2-1.5 wt % of Pt, 0-0.4 wt% of Pd, 0-0.4 wt % of Rh, 0-5.0 wt % of Ba, 40-90 wt % of CeO₂,9.8-59.8 wt % of Al₂O₃, and 0-20 wt % of an additive based on a totalweight of the CUC.
 7. The after treatment system of claim 6, wherein theadditive comprises at least one of La, Zr, Mg, and Pr.